1. Field of the Invention
The technical field is gas phase analytical instrumentation, and in particular, a focusing device for gas chromatography.
2. Description of Related Art
Gas chromatography is an analytical technique that separates compounds via gas-phase physicochemical processes. Samples comprised of mixtures of compounds are introduced into chromatographic system (sample introduction), vaporized (various means) if not gaseous already, moved by an inert gas stream (carrier gas) into and through a separation column or columns. Sample components separate from each other when they travel through the column at different speeds due to selective interaction with the column and its coating or packing (the stationary phase). Components eluting from the column are then detected by an appropriate detector.
Performance in gas chromatography is often reduced due to spreading of solute bands wider than their theoretically optimal widths. The optimal widths are defined by well understood relationships of column (or column packing) dimensions, carrier gas type (e.g., He, H2, N2) and flow rate, stationary phase type and thickness, and temperature program rates. Sample introduction devices (inlets) that transfer sample into the analytical column dictate the initial bandwidth of the sample. If sample introduction is slow, such as with splitless injection modes, headspace analysis, thermal desorption, etc., then some sort of focusing technique is required to narrow the input bandwidth sufficiently to be appropriate for the given analytical system being used for separation. As solute bands move through the column, they naturally spread further. The degree of spreading in the column is well modeled by known relationships. In some applications, it is advantageous to narrow the width of solute bands eluting from one column as it passes to another column, a detector, or other zone. Having a narrow initial bandwidth in the subsequent zone will often enhance chromatographic performance.
Various means for correcting for band spreading, that has occurred in one section, prior to release into another section have been developed. Methods for correcting band spreading or narrowing the bands are often referred to as focusing. Current focusing methods, however, all have disadvantages and limitations.
Focusing is typically accomplished through some combination of thermal focusing and solvent focusing. The migration of solutes slows approximately 2-fold for every 25° C. decrease in temperature. So, in simple terms, thermal focusing works based on the principle that solutes tend to “stick” to the stationary phase if temperatures are significantly lower than their elution temperatures. Temperatures at least 50° C. less than the elution temperatures is typically recommended, however, the lower the temperature, the less mobile the solute and the more effective the focusing.
Solvent focusing relies on re-condensation of evaporated solvent in the head of the column. This requires that the condensation zone be below the boiling point of the solvent. The lower the temperature below the boiling point of the solvent, the faster the condensation process and more confined the solvent (and therefore solute) zone. The condensed solvent acts to re-dissolve solutes that were evaporated in the chromatographic inlet. Evaporation expands the volume of a liquid several hundred times as it goes into the gaseous state. Re-condensation and re-dissolution reverse that effect, thereby reducing the volume several hundred times as the vapors enter the cooled zone.
Thermal focusing or “cold trapping” in GC is known. However there are limitations to the currently practiced trapping techniques. In one common implementation, the entire oven area of the instrument is cooled. This effectively turns the entire contents of the oven into a trap. This is useful for focusing broad bands that are caused by slow sample introduction (sample introduction device being outside the oven). However this implementation can not be easily be applied to focusing separated bands eluting from one column prior to passing them to another (multidimensional chromatography) and require large amounts of cooling and heating because the whole oven area and contents are temperature cycled.
Another common implementation is to direct cryogen (or another cooling gas) on a small section or column in the oven using a jet. The oven stays at whatever higher temperature it is set at, while the small section of column experiences a lower local temperature. Although more applicable to multidimensional chromatography, this approach usually consumes a large amount of coolant and at the same time requires more power from the oven heater to compensate for the cooling of the cryogen being added to the oven.
Another approach is to use electro-thermal devices in contact with a short section of column in the oven. This suffers from high failures and such devices have limited temperature cycling range (especially when in a heated oven environment).